To improve the effectiveness of energy harvesting, ocean Wave Energy Conversion (WEC) devices need a more efficient bi-directional air turbine. Specifically, Oscillating Water Column (OWC) systems offer unique challenges. As the water level rises and falls within an OWC wave capture chamber, the resulting compressed air stream will reverse direction with every wave cycle. When a pneumatically driven turbine is exposed to this reciprocating air flow, a standard axial flow turbine design would have a tendency to reverse its rotational direction. This is unacceptable if a turbine has been designated to be the Power-Take-Off (PTO) device to spin an electrical generator.
Several prior art approaches to resolve this problem have been patented. Perhaps, most notably, is a device described by Professor Alan A. Wells of Queens University in Belfast, Ireland. The Wells Turbine, as his device is commonly referred to, overcame the bi-directional air flow problem. His turbine consisted of a series of symmetrical airfoil blades that radiated out from a common shaft. The blades were configured to be perpendicular to the air stream so that the turbine would rotate in a single direction regardless of the direction of air flow.
The Wells blade configuration has a high angle of attack relative to the air flow. This introduces performance and efficiency issues with regard to lift and stall characteristics. Consequently, over the past three decades, WEC device developers and independent researchers have attempted to improve the operation of the Wells design.
Expensive and complex variable pitch and counter-rotating blade designs have resulted in modest gains in efficiency. The Wells design continues to be challenged by low efficiency, issues with low-speed operation, a small operating window, and an inability to self-start under many conditions.
In addition to the Wells Turbine, other OWC turbines currently in use include: (a) impulse turbines (analogous to a Pelton Wheel adapted for air) which use guide vanes and ducting to force the air stream into a unidirectional flow through the turbine, and (b) variable pitch axial turbines like the Denniss-Auld Turbine which rely on complex movable blades to adapt to the reversing flow every cycle.
Despite a large body of work from India and Japan attempting to refine the Wells Turbine, the design falls short of being a reliable and efficient PTO device. In consideration of the current state of technology and the need for efficient, cost-effective renewable energy, a new turbine design is described as follows: